The semiconductor diode laser in a p-n junction device which lases when a forward bias voltage of at least 1.5 volts is applied to the device. The voltage drives either holes or electrons or both across the p-n junction and when the holes and electrons recombine they emit light. For an instant before the holes and electrons recombine, they can be "stimulated" by light to emit more light coherently. This stimulated emission phenomenon is equivalent to providing amplification and is related to the first of two requirements for laser oscillation. Specifically, a first requirement is that there be sufficient gain or amplification of the light within the laser to overcome all losses. The second requirement for laser oscillation is an optical feedback mechanism. Optical feedback is provided in conventional diode lasers by simply "cleaving" the faces of the semiconductor crystal. These cleaves form plane parallel mirror-like surfaces which reflect a portion of the light back into the region of the p-n junction. The reflected light is amplified and the energy density within the laser continues to build up to produce the very intense laser beam.
Several problems have thus far tended to reduce the usefulness and versatility of the described "cleaved-faced" diode laser. First, these diode lasers often fail within tens to hundreds of hours of usage because of damage caused by the high intensity of the light incident on the cleaved mirrors. Secondly, and equally important, no means is known for integrating these diode lasers into an integrated optical system.
In patent application Ser. No. 761,105 filed Jan. 21, 1977 by the inventors of the subject application and entitled SEMICONDUCTOR LIGHT REFLECTOR/LIGHT TRANSMITTER, there is described a semiconductor device which is coupled by a waveguide structure to a diode laser source and which operates on interferometric principles to provide either light reflection or transmission and light amplitude modulation. That device provides a means of integrating a diode laser into an integrated optical circuit and has the advantage of electrical control which is not possessed by distributed feedback laser devices. However, it would be desirable to have an interferometric reflector as part of the laser active region so that the optical feedback mechanism is incorporated directly into the laser structure.